1. Field of the Invention
The present invention relates to an external cavity type of wavelength tunable semiconductor laser light source, and a method for tuning wavelength therefor, which are used, for example, in an optical measurement technical field.
2. Description of the Related Art
In order to use a semiconductor laser light source for an optical measurement technique, one of single mode oscillation type, tunable in wavelength and having a narrow spectral line width and a good stability of wavelength, is required.
As a conventional external resonator type of wavelength tunable semiconductor laser light source, for example, the ones shown in FIGS. 5 to 7 are known.
FIG. 5 is a block diagram showing a construction of a conventional general external resonator type of wavelength tunable semiconductor laser light source.
The external resonator type of wavelength tunable semiconductor laser light source comprises a semiconductor laser 4, a semiconductor laser drive circuit 10, lenses 5, 6 and 7, an optical isolator 8, a diffraction grating 2, an angle adjusting mechanism 63, a wavelength tuning drive circuit 61, a parallel movement mechanism 62, a position adjusting drive circuit 60 and the like.
The semiconductor laser 4 is of a Fabry-Perot type, and is provided with an anti-reflection film (AR coating) 4a on an end facet thereof. The semiconductor laser 4 emits a light beam from the both end facets according to the injection current given by the semiconductor laser drive circuit 10.
The light beam emitted from the end facet with the anti-reflection film 4a of the semiconductor laser 4 is changed to a collimated beam by the lens 5 to enter the diffraction grating 2.
The diffraction grating 2 is used as a wavelength selecting reflector, that is, it has a function of reflecting a light beam having a specific wavelength which is determined by the incident angle of the incident collimated light beam.
The diffraction grating 2 forms a resonator in cooperation with an end facet having no anti-reflection film of the semiconductor laser 4. Since the light beam having the specific wavelength which was selected by the diffraction grating 2 is reflected back to the semiconductor laser 4, a laser oscillation can be generated.
The lens 6 is placed on the exit optical axis of the side having no anti-reflection film of the semiconductor laser 4, in order to change the light beam emitted from the end facet of the semiconductor laser 4 to a collimated light beam. The emitted light beam which was changed to the collimated light beam enters the optical isolator 8.
The optical isolator 8 is for preventing a reflected light beam from an output fiber 100 side from returning back to the semiconductor laser 4. The light beam which was passed through the optical isolator 8 is condensed through the lens 7 and is introduced into the output fiber 100 as an output light beam.
The diffraction grating 2 is adjustable to have a desired angle with respect to the incident optical axis by the angle adjusting mechanism 63.
The angle adjusting mechanism 63 can be controlled by the wavelength tuning drive circuit 61, so that the diffraction grating 2 is rotated to have a desired angle and thereby a wavelength to be selected (Bragg wavelength) is desirably changed. Therefore, it is possible to tune the wavelength within the gain range of the semiconductor laser 4.
The diffraction grating 2 can be moved in parallel with the incident optical axis by the parallel movement mechanism 62. That is, by controlling the parallel movement mechanism 62 by the position adjusting drive circuit 60, the diffraction grating 2 is moved in a direction parallel with the optical axis of the resonator, so that the resonance wavelength of the resonator can be desirably changed.
By controlling the angle adjusting and the parallel movement, of the diffraction grating 2 at the same time by such mechanisms, it is possible to provide a continuous single mode scanning over a range of wavelengths without mode hopping.
In the structure of the construction of the external resonator type of wavelength tunable semiconductor laser light source shown in FIG. 5, it is extremely difficult to conduct a continuous scanning over a range of wavelengths without mode hopping by precisely drive-controlling both the angle adjusting and the parallel movement, of the diffraction grating 2 at the same time.
Because the angle adjusting mechanism 63 and the parallel movement mechanism 62 use a rotation stage and a linear movement stage and the like, a mechanical backlash may occur. Therefore, there is a problem that it is not possible to tune wavelength precisely.
Further, because use of a motor or the like, having precise gears built-in to control the mechanism precisely is required in addition to the rotation stage and the linear movement stage, there is a problem that a wavelength tuning mechanism becomes large in size.
As an example that the wavelength can be tuned without mode hopping by a simple drive control, a construction of a conventional external resonator type of wavelength tunable semiconductor laser light source of the so-called sine bar structure is shown in FIG. 6.
The external resonator type of wavelength tunable semiconductor laser light source comprises a semiconductor laser 4, a semiconductor laser drive circuit 10, lenses 5, 6 and 7, an optical isolator 8, a diffraction grating 2, an angle adjusting mechanism 63, a wavelength tuning drive circuit 61, a parallel movement mechanism 62, an arm 72, a contact table 73 and the like.
To structural members in FIG. 6, elements or the like corresponding to those shown in FIG. 5, the same reference numerals are attached, and the detailed explanation for them is omitted.
The parallel movement mechanism 62 is for adjusting the length of the external resonator. The parallel movement mechanism 62 enables movement of the diffraction grating 2 in parallel with the optical axis by controlling of the wavelength tuning drive circuit 61.
The angle adjusting mechanism 63 is for adjusting the angle of the diffraction grating 2 with respect to the optical axis. When the diffraction grating 2 is moved in parallel with the optical axis by the parallel movement mechanism 62, the angle of the diffraction grating 2 is changed through the arm 72 which can slide in the direction of the arrow while being in contact with the contact table 73, at the same time.
As described above, according to the sine bar mechanism, it is possible to adjust both the angle of the diffraction grating 2 and the length of the external resonator at the same time only by a drive control for linear movement by using the wavelength tuning drive circuit 61. Accordingly, it is possible to carry out a continuous scanning over a range of wavelengths without mode hopping easily.
There is a report that a continuous wavelength tunable width of 80 nm has been already obtained. There is a product of the external resonator type of wavelength tunable semiconductor laser light source in which such a sine bar mechanism is used.
However, because such the mechanism requires use of two stages, i.e., the rotation stage as the adjusting mechanism 63 and the linear movement stage as the parallel movement mechanism 62, the backlash of these stages or the like becomes a big problem. Further, because two stages are used, there is the problem that the wavelength tuning mechanism is enlarged, like the example shown in FIG. 5.
As an example of an external resonator type of wavelength tunable semiconductor laser light source which uses no linear movement stage, for example, the one having a rotation arm structure which is disclosed in Japanese Patent Application Publication (not examined) No. Toku-Kai-Hei 3 (1991)-279821 is known. The construction of the one is shown in FIG. 7.
The external resonator type of wavelength tunable semiconductor laser light source comprises a semiconductor laser 4, a semiconductor laser drive circuit 10, lenses 5, 6 and 7, an optical isolator 8, a diffraction grating 2, a wavelength tuning control member 83, a wavelength tuning drive circuit 81, a rotation axis 84, an arm-shaped mechanism 82 and the like.
To structural members in FIG. 7, elements or the like corresponding to those shown in FIG. 5, the same reference numerals are attached, and the detailed explanation for them is omitted.
The diffraction grating 2 is mounted on the nose of the arm-shaped mechanism 82 having the rotational shaft, that is, rotational center 84. The angle (rotation) of the diffraction grating 2 and the length of the external resonator are adjusted at the same time by rotating the arm-shaped mechanism 82 by the wavelength tuning control member 83.
The wavelength tuning mechanism having such a rotational arm is smaller in size than the one using the linear movement stage and the rotation stage. The drive control therefor is also simpler. There is a report that a continuous wavelength tunable width of about 1 nm (optical frequency tunable width of 130 GHz)is obtained, which is extremely small compared with the one which obtained in the sine bar mechanism shown in FIG. 6.
But the rotational shaft 84 needs to use bearings for rotation and the like. Therefore, problems are remained, that is, an occurrence of backlash at the bearing portion, the large-sized bearing portion, and the like.
As described above, in the structure of the conventional external resonator type of wavelength tunable semiconductor laser light source which uses the rotation stage and the linear movement stage as the angle adjusting mechanism and the parallel movement mechanism, respectively, because it is hard to avoid mechanical backlash, precise wavelength tuning is very difficult.
Further, because in order to control the mechanism precisely, use of a motor having precise gears built-in is required in addition to the rotation stage and the linear movement stage, there is the problem of a large-sized wavelength tuning mechanism.